Neurotransmitter Actions Katya Likhtik

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1Neurotransmitter Actions Katya LikhtikWhat is a neurotransmitter?Under what conditions and how are they released?What happens to the presynaptic and the postsynaptic neuron once they are released?

2The 18th and 19th C debate about the nature of communication in the nervous system: Electrical or Chemical??After a presynaptic neuron is stimulated the delay is about 0.3 ms for the postsynaptic neuron to respond. This is too long for electric transmission.If you stimulate the postsynaptic neuron , no response in the presynaptic one. Polarization of communication between neurons.Stimulation of presynaptic neuron may result in postsynaptic inhibition. Difficult to explain in terms of direct passage of electrical event.No relationship between the magnitude of the pre and postsynaptic electrical event.

4Not to say that there aren’t electrical synapses!!Gap junctions – cell to cell pores that allow ions and very small molecules to pass from the cytoplasm of one cell to the next.

5Before a substance can be called a neurotransmitter:Presynaptic terminal should contain a store of the substance (preferably in a sequestered form)Applying the substance to a postsynaptic cell should mimic the effects caused by stimulating the presynaptic terminalIf a drug is known to block a neurotransmitter, it should have the same effect on this transmitter if it’s applied exogenouslyA mechanism for the synthesis of this trasmitter must exist (including the appropriate precursors/enzymes in the terminal)A mechanism for inactivation of the transmitter must exist (catabolic enzymes for its degradation/ reuptake system, etc)

7What kinds of neurotransmitters are there?AMINO ACID transmittersGlutamateGABA (-aminobutyric acid)Most transmitters are small, water-soluble molecules containing amine and (in the case of amino acid transmitters) carboxyl groups. These chemical groups cause the transmitters to be ionized at physiological pH and thus reduces the probability of passing the blood-brain-barrier (BBB!).

8Acetylcholine (Ach)Present both in the central and peripheral nervous systems (CNS &PNS)Synthesized by the combination of AcetylCoA, which is a product of the Krebbs cycle in the mitochondria, and choline, which is obtained from food (egg yolk, legumes).In the PNS, it is the transmitter of the neuromuscular junction- between neurons and all types of muscle (cardiac/smooth/skeletal) and thus is responsible for muscle contraction.

9The monoamine neurotransmittersDopamine (DA) – concentrated in neurons of theVentral Tegmental Area (VTA) and in thesubstantia nigra of the basal ganglia. Importantfor motion, mood, reward, schizophrenia, etcNE – first discovered in the sympathetic branchof the autonomic nervous system. Cell groupscontaining NE found in the locus coerulus (LC),which projects all over the brain and partakes inThe sleep-wake cycle, attention, vigilance.EPI – sympathoexcitatory, found in the adrenalmedulla and in cell groups of the medulla (oblongata)

10Serotonin (5HT)First identified as an element found in the blood that aided its clotting and produced vasoconstriction (originating from “serum” + having an effect on muscle “tone” resulted in the name serotonin)5HT neurons found mostly in the raphe nuclei that are located in the brainstem and that innervate all major brain areas5HT has been found to be important for food intake, aggression, mood. It is manipulated by antipsychotic drugs. Variations of serotonin (slight changes in its chemical structure) result in hallucinogens such as LSD, ecstasy, mescaline.

12Vesicle transport to the terminal is via microtubulesThe synthesis of vesicles occurs in the cell soma, where they pinch off the Golgi apparatus and are transported to the terminal along microtubules in the axon.

13Vesicles are transported to the terminal via an active processVesicle transport requires ATP hydrolysis by a small molecular “motor” called kynesin.

14Loading neurotransmitters into vesicles at the terminalThe proton pump hydrolizes ATP and produces a proton gradient and membrane potential across the granule membrane. The amine transporter binds an uncharged amine and couples its transport INTO the vesicle with the H+ exit OUT of it. The amine then becomes protonated at the acidic pH of the vesicle and is stuck inside the vesicle.

15Vesicles aggregate at the presynaptic terminal at the synapseA synaptic cleft in the CNS separates the pre and postsynaptic terminals by about nmSynapses can be axodendritic, axosomatic and axoaxonicActive zones- areas of the presynaptic membrane that are sites of vesicle attachment and neurotransmitter release

16Gray’s Type I and Type II synapse structureNote the differences in localization of synapses, vesicle shape, the shape of the synaptic cleft.Type I - usually glutamatergic, excitatoryType II - usually GABAergic, inhibitory

17The structure of the neuromuscular junction (NMJ) in the PNSThe junctional folds found on the postsynaptic side of the NMJ increase the number of Ach receptors that are exposed to release of neurotransmitter, resulting in very efficient transmission

18Neurotransmitter Release is Ca++ dependentRelease of neurotransmitters is dependent upon the entry of Ca++ into the presynaptic terminal through voltage sensitive Ca++ channels, which are clustered near release sites.Voltage increases sufficient to open Ca++ channels usually occurs with the arrival of an action potential to the terminal. The terminal can also be stimulated with an electrode to increase voltage.TTX blocks Na+ channelsTEA blocks K+ channels

19Vesicle ExocytosisThe time between docking and exocytosis is less than 200 ms. In order for vesicles to dock and to be primed for release, ATP is necessary. Thus mitochondria are present at the terminal.

20The SNARE complex model of vesicular fusionAs vesicles fuse with the membrane the SNARE complex forms as a result of the close association of proteins found on the membranes of the vesicle and plasma membrane.Toxins, such as botulinum toxin and tetanus toxin, selectively cleave different proteins of the SNARE complex, preventing vesicle fusion and neurotransmitter release, thus causing paralysis. (Botox is also used in plastic surgeries to eliminate wrinkles of the forehead.)

21Vesicle endocytosis and recyclingAfter neurotransmitter release, the vesicular membrane is coated with the protein clathring, thus identifying it for recycling. The clathrin coated pits are transferred to the endosome, where the membrane is reused for new vesicles and refilled with neurotranmitter.

22Neurotransmitters are released in “quanta”Spontaneous miniature end plate potentials (MEPPs) are usually about 0.5mV. Stimulation of the presynaptic terminal causes post-synaptic end plate potential (EPP) amplitudes to be at multiples of the MEPP. With iontophoretic application of Ach, it was estimated that about 5000 molecules (or a quantum) of Ach are released synchronously into the synaptic cleft to generate a single MEPP.1950s, Katz et al., suggested that the packets of quanta of Ach correspond to neurotransmitter content of a single vesicle

23Removal of the neurotransmitter from the synaptic cleftTimely removal prevents desensitization of receptors and interference with new incoming signals.MechanismsEnzymatic degradation: AcetylcholinesteraseA nearly 100% efficient enzyme. Ach molecules are hydrolized as rapidly as they can diffuse into the active site. Forms acetate and choline.Breaks down 14,000 Ach mols/sec (1 moleucle in 70 µsec)

24Degradation of CatecholaminesMonoamine Oxidase (MAO) – an enzyme that catalyzes the neurotransmitter to its corresponding aldehyde. This form can be broken down further and excreted. MAO is located in the mitochondria of catecholaminergic terminals.Catechol-O-Methyl-Transferase (COMT) – methylates a hydroxyl group of the catechol nucleus. The products can then also be excreted.Antidepressant drugs act on these enzymes. E.g. Pargyline – a clinically effective antidepressant is an MAO inhibitor, increases DA levels in the CNS.

25The most common mechanism: Reuptake by transporter moleculesTransporter molecules are in the presynaptic nerve terminals and glial cellsTransporter molecules have binding constants of <25 mM.serotoninnorepinephrineDopamine cocaine and amphetamine prevent reuptake of DA and NEglutamateGABAglycinecholine (after acetylcholine is broken down, choline is taken into thepresynaptic nerve terminal for reuse)

26Postsynaptic actions of neurotransmitters are receptor dependentLigand-gated ion channels – activated by the biding of a neurotransmitterBinding of the ligand stabilizes the active conformation of the receptor, thereby opening an ion channel, which is created by the arrangement of four to five receptor subunits.e.g. Nicotinic receptor for Ach

27An example of a ligand gated ion channelThe nicotinic Ach receptor. It needs two molecules of Ach in order to open. It is permeable to cations. It is found in the NMJ, autonomic ganglia, hippocampus, thalamus, etc.

28G-protein mediated receptorsG-protein is a guanyl nucleotide-binding protein. In its inactive state it has a GDP bound to it, whereas in its active state it has a GTP bound to it. In its active state, the protein can interact with an effector system (such as an ion channel)

29Second messenger systemsThe G protein activates a second messenger which then activates any number of other mechanisms inside the cell, which can alter the intracellular calcium concentration, open/close channels, alter gene production

31The postsynaptic EPSPWhen the channel opens, both Na+ and K+ can flow through the channel. Based on the resting membrane potential (near -70 mV) and the equilibrium potentials for each ion, we can see that there is a tremendous potential difference for Na+, yet a very small differential for K+. As a result, at rest, acetylcholine triggers a rapid influx of Na+. This influx of Na+ leads to an excitatory postsynaptic potential

32The postsynaptic IPSPAn inhibitory postsynaptic potential is often mediated by the opening of chloride channels which cause an influx of Cl- into the cell

35Action of antidepressants and other drugs at serotonergic synapsesSSRI- selective serotonin reuptake inhibitors

36The Lag Time Enigma and Antidpressant ActionA substantial lag between the time an antidepressant is administered and relief of symptoms, suggested that changes occur post and not presynaptically.Prompted the hypothesis that pathogenesis and treatment of depression involves a plasticity/adaptation in relevant neuronal pathways.The current model suggests second messenger systems that upregulate postsynaptic signaling cascades and result in an increased production of BDNF (brain derived neurotrophic factor)

38Pharmacotherapy of Anxiety“Because I associated the attacks with driving, I reduced my driving to pure necessity. Eventually because I feared another attack and the consequent embarrassment, I avoided all but essential social contact. Ultimately I was frightened to leave my own home.”An estimated 19 million Americans suffer from anxiety disorders (AADA - Anxiety Disorders Association of America)Benzodiazepines: Acute anxiety, generalized anxiety disorder (GAD), panicAntidepressants: Generalized anxiety disorder, panic, obsessive compulsive disorder

39Pharmacotherapy of AnxietyDiazepam - a benzodiazepine that is effective at treating GADWorks via the GABAa channel (permeable to Cl-). It increases the affinity of GABA for the receptor, thus increasing the Cl- conductance and the hyperpolarizing current.High concentrations of GABA receptors are found in the limbic system (“the emotional system”)